This invention relates to a powered conveyor system for moving loads from an input position to an unload position on the conveyor, and more particularly, to a powered conveyor system which can be used as a storage rack for high density storage of a number of loads between the input position and the unload position of the conveyor.
In the material handling industry items which are to be moved and stored are normally mounted on pallets. Generally speaking, there are two common and widely used types of systems for moving pallets with a positive, forward motion: gravity flow systems and powered conveyor systems. There are many, different designs of both types of systems used for commercial and industrial purposes.
Gravity flow systems are most commonly used because they are more easily adapted to end users' needs, are reliable first-in, first-out, or FIFO, high density storage systems and are considerably more inexpensive than powered conveyor systems. Gravity flow systems must be inclined to allow pallets loaded on them to flow forward on wheels or rollers or on other low friction supports for the pallets. The slope of the incline for each system is usually between 5/16ths of an inch per foot of horizontal run and as much as 1 inch per foot of horizontal run.
Gravity flow systems have several problems. Among the most significant is the need to balance the slope required to allow enough break away momentum for the range of anticipated loads, with the need to keep the loaded pallets flowing at a controlled, safe speed. Most manufacturers of gravity flow systems recommend that the loads be kept as uniform as possible and that the weight of the lightest load should be no less than about half the weight of the heaviest load. If the range of load weights becomes too great, the greater incline necessary to move lighter loads causes heavier loads to travel much faster. This increases the risk that heavier loaded pallets will travel at an unsafe speed and will be involved in accidents, causing damage and/or injury.
Another problem faced by gravity flow systems is the height required at their input ends due to the slope of the system. The longer the run of a system, the higher the input end must be to allow for the required slope. For example, a gravity flow system designed to store 12 loaded pallets which are each 4 feet deep requires about 50 feet of horizontal run. If the slope of the incline is 1/2 inch per horizontal foot, then the input end must be 25 inches higher than the unload end of the system. As a result, the end user may be limited in height with respect to the number of rows of storage which can be stacked vertically using a gravity flow conveyor.
Other problems with gravity flow systems include a high line stack weight of each system, that is the amount of force being applied to the pallet first in line to be removed due to weight of the loads on the pallets behind it. As the line stack weight becomes greater it decreases the safety of personnel who free pallets that hang up in the system. Additionally, gravity flow system performance depends greatly on the type and physical conditions of pallets being used. Pallets may be made from a variety of materials such as wood, metals, and various plastics. Each type of pallet material presents its own unique problems with regard to the ability of pallets to flow on a gravity flow conveyor system. The physical condition of the pallets being used may also cause additional problems. Powered conveyor systems are often used to avoid these problems.
Powered conveyor systems are more reliable but not as flexible as gravity flow systems. Because powered conveyor systems are powered by motors and the speed of pallets or other loads on these systems can be controlled more easily, powered conveyor systems are reliable and readily transfer pallets from place-to-place.
However, powered conveyor systems have several undesirable qualities when being used as storage rack systems. For example, powered conveyor systems which move loads by driving a sequential series of individual rollers or conveyor belts often require side mounted sections which house motors, gears, sprockets and other drive mechanisms. The use of these drive sections makes it difficult to use them as a storage rack system which is more than one level high. Since generally speaking a separate drive section and its associated motor and housing must be used for each individual powered conveyor, no more than two conveyors can be installed side by side, with a maintenance lane between adjacent sets of conveyors. This results in a two wide conveyor system which is one level high, thereby wasting valuable warehouse space.
Another problem with some powered conveyor systems is that due to the high cost of each powered section, not all sections of the conveyor are powered. The sections which are powered generally do not exceed twelve feet in length. In contrast, the non-powered sections are normally much longer, with some being several sections longer than the powered sections being used. Pallets on the non-powered sections of the conveyor have to be pushed onto the powered sections as subsequent loads are moved on the powered sections, and they are not easily removed when the system must be emptied. As a result, loads may not be as tightly packed together as is common with gravity flow systems. It is not uncommon to have some powered conveyor systems leave large gaps between adjacent loads, even when the systems are considered to be full. This, too, wastes valuable warehouse space and leads to an inefficient movement of loads to the unload position of these systems.
Other powered conveyor systems move loads through the use of complex mechanical equipment and computer tracking systems. These powered conveyor systems become too expensive to be used as a general storage system and are normally designed to meet the particular needs of a specific application. A system of this type is shown as U.S. Pat. No. 4,732,524 Suominen. Even powered roller and conveyor belt systems can cost three to four times the cost of a gravity flow system.
Some powered conveyor systems have been designed, at least in part, to solve some of these problems by using transport carriages to move loads. The transport carriages are mounted on rollers which roll within a set of channel irons upon which loaded pallets are placed. These transport carriages each move loaded pallets by lifting them off the channel irons one at a time, moving each lifted load to an open area in the row of the storage rack system in which the transport carriage is operating. These systems require separate mechanisms to cause the transport carriages to lift loads and to move the loads horizontally. Many of these systems also use computer systems to track the locations where the loads have been set by the transport carriages to enable these systems to move loads from an input position to an unload position of the conveyor in an automated manner. These separate mechanisms and computer systems add to the complexity and cost of this type of powered conveyor system. These systems have the disadvantage that each transport carriage can only move one load at a time. As a result, loaded pallets cannot flow steadily from the input position of this type of powered conveyor system to its unload position.